They could simply decouple the apps from the OS, like Android. Safari and Mail.app do not need to be tied to OS releases.

Android OS releases do not get nearly as much fanfare because a lot of core functionality is decoupled from the OS: app store(s), mail clients, browsers, todo lists, even the text messaging apps and some security frameworks.

Which of course is exactly why Apple keeps everything in one big release. They want lots of press talking about the brand new features.